Micro lenses have attracted extensive attention because it is one of the most important components in the microoptics field for optical fiber coupling, optical parallel processing, optical interconnection, adaptive optics, beam steering and optical biomedicine. Many kinds of methods and materials have been proposed to fabricate micro lens.
Liquid crystal (LC) devices have been widely used in a variety of applications including display, shutter, mirror, and grating. For example, Tsubota et al. in U.S. Pat. No. 5,499,127 describes a LC device with a display area and U.S. Pat. No. 5,699,133 to Furuta describes the use of twisted nematic liquid crystal (TNLC) as a shutter. Another use of liquid crystals as cells is disclosed in U.S. Pat. No. 5,919,606 to Kazlas et al. A polymer dispersed liquid crystal (PDLC) device with a polymer concentration of more than 65% is disclosed in U.S. Pat. Publication No. US2002/0097355 A1 to Kralik et al. In addition, liquid crystal (LC) devices have also been proposed for application as electrically tunable micro lens due to the ability of controlling the phase of light passing through them; see for example, U.S. Pat. No. 4,572,616 to Kowel, U.S. Pat. No. 5,071,229 to Oaki et al., U.S. Pat. No. 5,150,234 to Takahashi et al. and U.S. Pat. Publication No. 2002/0181125 A1 to Nishioka. Cylindrical and circular LC lenses with variable focal length have been produced using slit-patterned and circular-hole-patterned electrode structures, respectively. Two papers have been published: 1. “Cylindrical liquid crystal lens and its applications in optical pattern correlation systems”, published by He et al in the Japanese Journal of Applied Physics, Vol. 34, No. 5A, May 1995, pp. 2392-2395, and 2. “Transient properties of a liquid crystal micro lens”, published by Ye and Sato in the Japanese Journal of Applied Physics, Vol. 40, No. 11, November 2001, pp. 6514-6521. However, there are some problems with the liquid crystal devices. The major disadvantage is that these devices are sensitive to the polarization of the incident light. Therefore, a linearly polarized light is usually required to avoid the polarization dispersion. In addition to the loss of at least half of the incident light, the requirement of linearly polarized light is also an obstruction to applying liquid crystal lens in the micro-optics field, especially for fiber-optic communication, because the polarization of the light passing through a fiber is unpreserved.
To overcome the polarization problem, J. S. Patel published a paper “Polarization insensitive tunable liquid crystal etalon filter” in Applied Physics Letters, Vol. 59, No. 11, September 1991, pp. 1314-1316 reporting on a double crossed-layer liquid crystal device. However, the fabrication process is complicated and the light must be split into two beams passing through the cell simultaneously. In 2002, H. W. Ren and S. T. Wu published a paper on the gradient refractive index nanoscale polymer-dispersed liquid crystal (GRIN PDLC; Applied Physics Letters, Vol. 81, 3537-9 (2002)). Due to the nanoscale LC droplets involved, the GRIN PDLC lens is independent of light polarization. However, the nanoscale PDLC droplets are difficult to be reoriented by the electric field so that the required voltage is quite high (˜100V). In 1991, J. S. Patel et al published a paper “Electrically tunable and polarization insensitive Fabry-Perot etalon with a liquid-crystal film” in Applied Physics Letters, Vol. 58, pp. 2491-2493 (1991). The authors observed the polarization-independent phenomenon in a Fabry-Perot cavity with a 90° twisted nematic (TN) LC. However, this paper did not teach how to make a tunable electronic lens. To make such a lens, a gradient refractive index has to be formed.
In this invention, we develop methods for achieving a gradient refractive index profile in a polymer-network approximately 90° TN-LC cell for tunable electronic lens. Such an electronic lens is independent of light polarization and has a relatively low (<20V) operating voltage.